Cost-effective, high-performance photovoltaic devices have been under intense investigation. Recently, a novel photovoltaic material, methylammonium lead halide (MAPbX3) perovskite, has attracted much attention due to its unique potential in attaining increased power conversion efficiencies, while being suitable for use in low-cost solution-based manufacturing. Although ideal optical and semiconducting properties of MAPbX3 perovskite have been demonstrated, one fundamental limitation of such materials is that of unbalanced electron-hole diffusion length. In particular, an electron beam-induced current (EBIC) study showed that the effective diffusion length of electrons is shorter than that of holes (Leff,e−/Leff,h+<1) in methylammonium lead iodide (CH3NH3PbI3)-based perovskite hybrid solar cells (pero-HSCs), and as a result, such solar cells have an electron extraction process that is less efficient than that of the hole extraction process. Having an identical electron and hole diffusion length is critical to achieve equivalent carrier extraction efficiency in a symmetric device structure, which determines the final device efficiency. Therefore, to balance the efficiencies of the two types of charge carrier extraction processes (i.e. electron and hole extraction), and to restrain or limit the charge carrier recombination process, a separate conduit is needed to assist electrons that are being extracted from the perovskite material to the front electrode of the solar cell.
To achieve these performance requirements, two state-of-the-art device architectures have been developed. One device architecture is a meso-superstructured solar cell (MSSC), whereby a meso-structured TiO2 or Al2O3 are utilized as an electron transport layer (ETL) and insulating scaffold, respectively. This ensures that effective charge separation occurs via an enlarging interfacial area, and as a result, a dramatically enhanced power conversion efficiency (PCE) of over 15%, from solution processed perovskite solar cells has been able to be achieved in some circumstances. However, the relatively low electrical conductivity of TiO2 brings an additional charge transport resistance (RCT) within the TiO2 itself. In addition, the commonly used mesoporous metal oxides require the utilization high temperature sintering manufacturing techniques, which undesirably makes such perovskite-based solar cells incompatible with large-scale processing.
The second device architecture used to achieve balanced charge carrier extraction is planar heterojunction (PHJ) architecture. In PHJ architecture, a solution-processed, highly conductive n-type material, such as fullerene, is introduced into the electron transport layer (ETL). The higher electrical conductivity ensures a higher efficiency in the electron extraction process and less energy losses in the transfer process. However, the electron extraction efficiency of such devices is still not high enough due to the insufficient interface of the perovskite and fullerene in the PHJ, thereby inhibiting further increases in device performance. Moreover, in most PHJ structures, the commonly coarse surface of solution-processed perovskite films, generally results in an inferior contact with the fullerene, further deteriorating electron extraction. In addition, the rough surface morphologies of the perovskite film also introduce shunts into the solar cell, which leads to a large leakage current in the device, as well as a low fill factor (FF).
Therefore, there is a need for a perovskite-hybrid solar cell (pero-HSC) that has improved electron extraction performance. In addition, there is a need for a perovskite-hybrid solar cell (pero-HSC) that has a planar heterojunction (PHJ) that has improved contact between the perovskite and fullerene. There is also a need for a solar cell in which polar ethanol is used as a solvent for methylammonium iodide (MAI) precursor solution to give a homogeneous and pinhole-free CH3NH3PbI3 perovskite film, which has reduced roughness, and thereby improved contact between the perovskite layer and the fullerene layer. For example, in some embodiments, the resulting pero-HSCs demonstrated both enhanced short circuit current (JSC) and FF of 17.31 mA/cm2 and 77.2%, respectively, resulting in a corresponding power conversion efficiency (PCE) of 11.45% for example. There is yet another need for further improvements in electron extraction efficiency through simultaneously increasing the electrical conductivity of electron transport layer (ETL) and enlarging the interfacial area between the ETL and the perovskite layer, by using a bulk hererojunction (BHJ) composite of perovskite:fullerene, whereby a water-soluble fullerene (ws-fullerene) is mixed with an MAI precursor in ethanol. Consequently, in some embodiments of the present invention, the enlarged interface between the perovskite and the n-type material gives rise to an increased JSC of 19.41 mA/cm2 and an increased FF of 81.6% in the resulting BHJ pero-HSCs, corresponding to an enhanced PCE of 13.97%.